Electrically isolated signal path means for a physiological monitor

ABSTRACT

A circuit for providing electrical isolation between a patient and physiological monitoring apparatus comprising an amplifier, having applied to the input thereof a physiological signal from the patient, and a first circuit branch including an isolation transformer connected between a modulator and a demodulator for connecting a d.c. voltage source of relatively low magnitude to the amplifier. Connected to the amplifier output is a second circuit branch comprising an isolation transformer connected between a modulator and a demodulator, and the output of the demodulator is coupled to the input of physiological monitoring apparatus. Each of the isolation transformers includes an extremely low capacitance between the primary and secondary windings thereof to present an extremely high reactance to low frequency, including line frequency, alternating current and each is capable of withstanding relatively high voltages without breakdown.

[ Sept. 12, 1972 ELECTRICALLY ISOLATED SIGNAL PATH MEANS FOR APHYSIOLOGICAL MONITOR Primary Examiner-William E. Kamm Attorney-Christel& Bean [57] ABSTRACT [72] Inventors: Benjamin H. Weppner, Snyder; Leo

' P. Lefebvre, Tonawanda; Victor R. A circuit for providing electricalisolation between a Miller, Clarence, all of NY. patient andphysiological monitoring apparatus com- [73] Assignez MemendcreatbatchEectmnics, prising an amplifier, having applied to the input hm Clarencethereof a physiological signal from the patient, and a first circuitbranch including an isolation transformer Flled! 9, 1970 connectedbetween a modulator and a demodulator 211 App] 79,51 for connecting adc. voltage source of relatively low magnitude to the amplifier.Connected to the amplifier output is a second circuit branch comprisingan '8 "128/ 128/21 isolation transformer connected between a modulator58] Fie'ld B 2 08 R and a demodulator, and the output of the demodulator128/2 1 A /9 b B 6 is coupled to the input of physiological monitoringapparatus. Each of the isolation transformers includes an extremely lowcapacitance between the primary and 5 References Cited secondarywindings thereof to present an extremely high reactance to lowfrequency, including line UNITED STATES PATENTS frequency, alternatingcurrent and each is capable of 2,673,559 3/1954 Fawcett ..l28/2.06 Bwithstanding relatively high voltages without break- 3,52l,087 7/1970Lombardi ..128/2.l R down- 3,527,984 9/1970 Flanagan et a1 ..3l7/9 AC3,587,562 6/1971 Williams ..128/2.06 R 16 Clams 4 Drama FOREIGN PATENTSOR APPLICATIONS 975,373 11/1964 Great Britain ..l28/2.06 R

57 5 741 15 72 i7 5 i 4 5 MODULATOR J2 DEMODULATGR DE MODULATORMODULATOR j PATENTEDsmz m2 3.690.313

MODU LATOR D E MODU LAT'OR INVENTORS.

ATTORNEYS.

ELECTRICALLY ISOLATED SIGNAL PATH MEANS FOR A PHYSIOLOGICAL MONITORBACKGROUND OF THE INVENTION This invention relates to physiologicalmonitoring apparatus and, more particularly, to electrical isolation ofa patient being monitored by such apparatus.

Modern medicine is experiencing widespread use of electricalphysiological monitoring apparatus wherein electrical signals derivedfrom the patient indicative of his physiological behavior, such aselectrical waveforms produced by various body organs, are displayed andprocessed electrically. In recent times there has developed an increasedawareness of the potential hazard'of electrical shock from suchapparatus, and coupled with this response from doctors, patients andregulatory agencies that the necessary precautions be taken in theconstruction and operation of such apparatus to insure patient safety.

The primary potential electrical hazard associated with presentapparatus is from leakage current, which is current applied to thepatient undergoing examination either by the monitoring apparatus or anelectrical fault related thereto. In particular, it is possible withsome types of existing apparatus for the patient along with theequipment to become part of a ground path for electrical current from anexternal source or for current to leak from the apparatus to thepatient. This problem is' compounded in some apparatus wherein thepatient is connected electrically to the equipment ground or electricalreference point.

SUMMARY OF THE INVENTION .It is, therefore, an object of this inventionto provide an improved circuit for electrically isolating a patient fromphysiological monitoring apparatus.

- It is a more particular object of this invention to provide such acircuit for protecting a patient from leakage currents from an externalsource or from the physiological monitoring apparatus.

It is a further object of the present invention to provide such acircuit wherein the patients electrical activity is floated relative tothe electrical ground of the physiological monitoring apparatus.

The'present invention provides a circuit for electrically isolating apatient from physiological monitoring apparatus wherein physiologicalsignals derived from the patient are amplified and transmitted through apath to the input of physiological monitoring apparatus. The circuitincludes another path connecting the circuit amplifier to a dc. voltagesource of relatively low magnitude. Both paths include isolation meansfor isolatingelectrically the patient from the monitoring apparatus andfrom the dc. source as well as external sources. Each of the isolationmeans includes an extremely low capacitance to present an extremely highreactance to low frequency and line frequency alternating current aswell as being capable of withstanding relatively high voltages withoutbreakdown.

The foregoing and additional advantages and characterizing features ofthe present invention will become clearly apparent from a reading of theensuing detailed description together with the included drawing wherein.

BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 is a block diagram of acircuit for providing electrical isolation between a patient andphysiological monitoring apparatus according to the present invention;

FIG. 2 is a schematic diagram of a preferred form of the circuit of FIG.1;

FIG. 3 is a schematic diagram of a conventional isolation transformerhaving an earth ground shield; and

FIG. 4 is a schematic diagram of an isolation transformer according tothe present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT FIG. 1 shows in blockdiagram form a circuit 10 according to the present invention forproviding electrical isolation between a patient and physiologicalmonitoring apparatus. Circuit 10 comprises an amplifier 11 having a pairof input terminals 12, 13 and an output terminal 14. A physiologicalsignal is obtained from a patient by electrodes attached to portions ofthe patients body (not shown) and applied by means including leads 15,16 to the amplifier input terminals 12 and 13, respectively.

Circuit 10 further comprises means defining an electrical path forconnecting amplifier 11 to a dc. voltage source of relatively lowmagnitude. A dc. voltage source preferably delivering about 12 volts isdesignated generally at 20 in FIG. 1, one terminal of which is connectedto earth ground. The other terminal of source 20 is connected by meansof a lead 23 to the input terminal of a modulator circuit indicatedgenerally at 25. The electrical path or circuit branch connecting source20 to amplifier 11 further comprises a demodulator circuit indicatedgenerally at 28, the output of which is connected by means of a lead 30to amplifier 11. The path also includes isolation means in the fonn ofisolation transformer 32 having a primary winding 33 and a secondarywinding 34 for isolating electrically the patient from d.c. source 20 orany other external electrical source which might become connected orcoupled to this path. Transformer primary winding 33 is connected bymeans of leads 35, 36 to the output of modulator 25. Similarly,transformer secondary winding 34 is connected by means of leads 37, 38to the input of demodulator circuit 29. Transformer 32 is constructed toinclude an extremely low capacitance between primary and secondarywithout earth ground shielding to present an extremely high reactance tolow frequency and line frequency alternating currents in this path aswell as to be capable of withstanding relatively high voltages appliedacross primary and secondary windings without breakdown.

Circuit 10 further comprises means defining a signal transmission pathfor connecting the output of amplifier 11 to the input of physiologicalmonitoring apparatus. Output terminal 14 of amplifier 11 is connected bymeans of lead 40 to the input terminal of a modulator circuit designatedgenerally at 41 in FIG. 1. The signal transmission path furthercomprises a demodulator circuit 44, and the output signal fromdemodulator 44 is coupled by means of a lead 45 and amplifier 46 to acircuit output terminal 50 which, in turn, is connected to the input ofphysiological monitoring apparatus (not shown). The latter can comprise,for example, a standard chart recorder, an oscilloscope', or othersimilar apparatus for displaying and, in some instances, electricallyprocessing the physiological signals. The signal transmission path orcircuit branch also comprises isolation means in the form of isolationtransformer 51 having a secondary winding 52 and primary winding 53.Secondary winding 52 is connected by means of leads 54 and 55 tocorresponding input terminals of demodulator circuit 44. Likewise,primary winding 53 is connected by means of leads 56 and 57 to theoutput terminals of modulator circuit 41. Isolation transformer 51, likeisolation transformer 32, is constructed to include an extremely lowcapacitance between primary and secondary windings without earth groundshielding to present an extremely high reactance to low frequency andline frequency alternating currents which may be present in the signaltransmission path and to be capable of withstanding relatively highvoltages applied across windings 52 and 53 without breakdown.

Circuit further comprises means including lead 60 connecting the carrieroutput of modulator 25 to the signal transmission path together withmeans including lead 61 connecting the carrier input of demodulator 28to another point in the signal transmission path for synchronizing theoperation of the path connecting source .20 to amplifier 11 with theoperation of the signal transmission path as will be described in detailpresently. Bias voltage for amplifier 46 is obtained through lead 62from d.c. source 20. It is apparent, therefore, that the electricalground or reference level for the output signal appearing on terminal 50is earth ground.

Fig. 2 shownin detail a preferred construction for the circuit 10 ofFIG. 1. Modulator 25 comprises first and second transistors 70 and 71,the collector terminals of which are connected to corresponding ends ofprimary winding section 33a of isolation transformer 32. The baseterminals of transistors 70, 71 are connected through resistors 72 and73, respectively, to primary winding sections 33b and 330, respectively,of transformer 32. The transistor emitter terminals are connected tothe. other sides of the winding portions 33b and 330. A smoothingcapacitor 74 is connected in parallel with winding portion 33a, and asignal or voltage level input to modulator 25 is applied to the midpointof winding 33a by a lead 75 connected thereto and to the base terminalof transistor 70 through a resistor 76.

Source delivers a d.c. voltage of a relatively low magnitude, preferablyabout 12 volts. When the voltage of source 20 is connected to the inputof modulator through lead 23 and a resistor 77, this voltage is appliedfrom a divider comprising resistor 72 and resistor 76 to the baseterminal of transistor 70 to turn that transistor on thereby causing aflow of current in one direction through primary winding portion 33a.The relative magnitudes of resistors 72 and 76 are selected to insurethat the 12 volt input will turn on transistor 70. Transistor 71initially is nonconducting. The turning on of transistor 70 andconsequent flow of current through winding portion 33a, by virtue of theinductive coupling induces a flow of current in winding portions 33b and330. The sense of the winding portions 33b and 33c is such as to causecurrent flows in respective directions which turn transistor off andturn transistor 71 on. The conduction of transistor 71, in turn, causesa flow of current through primary winding portion 33a in an oppositedirection and again induces a flow of current in the winding portions33b and 33c. As a result, an alternate reversal of current is producedin primary winding portion 33a which induces an alternating voltage inthe transformer secondary winding 34. The frequency of alternation oroscillation is a function of the voltage input to modulator 25 and ofthe characteristics of transformer 32. According to this illustration,the frequency of oscillation is selected to be about 3,000 cycles persecond.

Modulator circuit 25 functions to convert the d.c. voltage input to aperiodic, i.e., pulsating or alternating, voltage signal availableacross output leads 37, 38. Circuit 25 thus functions to chop the d.c.voltage input applied thereto, and it is in this general sense that theterm modulator is employed to designated circuit 25 and circuit 41 aswell.

The modulated signal voltage present on secondary winding 34 oftransformer 32 is applied through leads 37, 38 to the input ofdemodulator circuit 28. Circuit 28 includes a conventional dioderectifier network, which is connected through an RC filter network forsmoothing the rectified signal to a pair of leads 85, 86 for connectionto appropriate portions of amplifier 11 as will be described presently.In addition, a pair of Zener diodes can be connected in series acrossthe output of the smoothing filter for overvoltage protection betweenleads 85, 86.

Physiological signals derived from electrodes attached to portions of apatients body are applied through leads 15, 16 to the input of network90. In addition, a reference electrode can be attached to a reference orground potential portion of the patients body and connected through alead 17 to the network 90. For example, in a typical electrocardiogramexamination, lead 15 would be connected to an electrode on the patientsleft arm, lead 16 connected to an electrode on the patients right arm,and lead 17 connected to an electrode on the patients right leg. Network90 is a protective network for circuit 10 and the monitoring apparatus,and it includes inductive and capacitive components to trap or filterout spurious radio frequency signals and further includes neon bulbs anddiodes to prevent a defibrillation electrical pulse applied to thepatient during cardiac arrest from damaging the equipment.

The output of network 90 is connected to the input of amplifier 11which, according to this illustration of the present invention,comprises three differential amplifiers 93, 94 and 95. Alternatively,circuit 10 could include a five lead input wherein each lead isconnected to a corresponding one of five amplifiers. The physiologicalsignals from network 90 are applied through leads 96 and 97 tocorresponding input terminals of amplifiers 93 and 94. A referencepotential line for network 90 and amplifier 11 is indicated at 98, andis coupled through lead 17 to the patients electrical referenceelectrode and to various other portions of circuit 10 as will presentlybe described.

The amplified physiological signals are available on amplifier outputterminal 14 which is connected through a resistor 100 to the input ofmodulator 41 in the signal transmission path of circuit 10. Modulator 41includes a transistor 101 of the MOS, field effect type having acontrol'or gate terminal 102 which is connected through-a lead 103 and aresistor 104 to lead 61 connected to secondary winding 34 of isolationtransformer 32. Transistor 101 thus is turned on and off at a rate equalto the frequency of modulator 25 which, according to this illustrationof the present invention is 3,000 c.p.s. A drain terminal 105 oftransistor 101 is connected to resistor 100, and a source terminal 106of transistor 101 is connected to reference potential line 98. Thevoltage at drain terminal 105 thus is driven to the reference level atthe rate of the frequency of modulator 25 by virtue of the fact thattransistor 101 is connected in controlling relation between these twovoltage level points and in controlled relation to modulator 25. As aresult, the amplified physiological signals are chopped or modulated bythis operation of transistor 101. A'substrate terminal 107 of transistor101 is connected through a resistor 108 to a low negative voltagesource.

The modulated signal is applied to primary winding 53 of transformer 51,with a resistor 109 in combination with resistor 1 00 functioning as avoltage divider for this signal. This signal appliedto transformerwinding 53 is single-ended. Due to the action of transformer 51 thesignal appearing on secondary winding 52 is double-ended, that is amirror image of the signal applied to winding 53. The double-endedsignal from transformer winding 52 is applied through a resistor 111 todemodulator 44, including a transistor 112 in which the mirror portionsare removed.

The base terminal of transistor 112 is connected through a resistor 113and lead 60 to primary winding 33 oftransformer32, and as a result,transistor 112 is turned on and off at a rate equal to the frequency ofmodulator 25 which, according to this illustration of the presentinvention, is 3,000 c.p.s. The emitter terminal of transistor 112 isconnected to earth ground, and the double-ended signal appearing at thecollector terminal of transistor 1 12 is grounded at a rate equal to themodulator frequency. Transistor 112 thus is connected in controlledrelation to modulator 25 and in controlling relation between transformerwinding 52 and the remainder of the circuit. As a result, the signal isconverted back to the single ended form and is applied through avariable, gain adjustment resistor 114 and a fixed resistor 115 to theinput of amplifier 46. A network comprising resistor 116 and capacitor117 connected across the input and output of amplifier 46 functions toremove the 3,000 c.p.s. chopping signal component. The signal appearingon output terminal 50 is a substantial replica of the amplifiedphysiological signal at the output 14 of amplifier 11.

The output signal on terminal 50, which is connected to the input ofphysiological monitoring apparatus, is referenced to the equipmentground by virtue of the connection of amplifier 46 through lead 62 tod.c. source 20. It will be noted that the electrical reference potentialof the patient, connected to circuit by lead 17, is connected throughline 98 to windings 34 and 53 of transformer 32 and 51, respectively,and hence isolated or floated from the equipment electrical grounds suchas those of do. source 20 and of the physiological monitoring apparatus.In addition, while the three electrode system of FIG. 2 is preferred insome monitoring procedures, lead 17 can be eliminated to provide a twoelectrode, differential system. Isolation transformers 32 and 51 stillwould serve to isolate or float the patient relative to the equipmentgrounds and in a manner preventing the flow of leakage current.

Isolation transformers 32 and 51 each are constructed to include anextremely low capacitance between primary and secondary windings withoutearth ground shielding to present an extremely high reactance to lowfrequency and especially line frequency alternating current as well asto be capable of withstanding relatively high voltages between primaryand secondary without breakdown. In particular, transformer 32 isconstructed so that the capacitance between primary winding 33 andsecondary winding 34 without earth ground shielding, according to thepresent'illustration, is approximately 12 picofarads. In addition,transformer 32 is constructed to have a dielectric withstanding voltageof approximately 7,000 volts rms at 60 cycles measured between thesecondary winding 34 to the primary winding 33. The dielectricwithstanding voltage between primary winding portion 33a and portions33b and 330 is about volts rms. In addition, with 2 volts at 400 cyclesapplied to primary winding portion 330 the following voltages shall bemeasured: 1.0 volt plus or minus 1 percent from the midpoint of windingportion 33a to the either of the other winding terminals, 0.248 voltsplus or minus 3 percent between the winding portions 33b and 330, and1.39 volts plus or minus 3 percent measured between the midpoint andeither terminal of winding 34. These various operating characteristicsof isolation transformer 32 which are required for the illustratedoperation of circuit 10 are of course obtainable by means of knowntransformer design procedures and determined by such factors as wiresize, number of turns in the windings, core material, and insulation asis readily apparent to those skilled in the transformer design art.

Isolation transformer 51 according to the present illustration isconstructed to have a capacitance between the secondary 52 and theprimary 53 windings without earth ground shielding of approximately 12picofarads. The dielectric withstanding voltage is approximately 7,000volts rrns at 60 cycles between the primary and secondary windings. Inaddition, with 12 volts at 3 kilo cycles applied to secondary winding52, the voltage at primary winding 53 shall be about 12 volts. With 12volts at 3 kilocycles applied to secondary winding 52 and with theprimary winding 53 open circuited, the transformer input current shallequal about 6 milliamperes. As with isolation transformer 32, thesecharacteristics of transformers 51 which are required for theillustrated operation of circuit 10 are obtainable through knowntransformer design techniques by controlling such factors as wire size,number of turns of the windings, core material and insulation as isreadily apparent to those skilled in the transformer art.

FIGS. 3 and 4 illustrate further a distinguishing characteristic ofisolation transformers 32 and 51 as compared with a common type ofisolation transformer provided with an earth ground shield. Inparticular, V

136 between secondary winding 132 and shield 133.

Transformer 130 can be designed whereby capacitance 134 between primarywinding 131 and secondary winding 132 has an extremely low value. As aresult, transformer 130 can provide satisfactory common mode rejection.Transformer 130 is not desirable, however, for isolating a patient fromleakage currents because a leakage current path exists from earth groundconnected to shield 133 through either stray capacitance 135 and 136 tothe corresponding transformer windings 131 or 132. In other words, weretransformer 130 included in an isolation circuit with primary winding131 connected through an amplifier and electrodes to a patient, andsecondary winding 132 connected to the monitoring equipment, a leakagecurrent path exists from earth ground connected to shield 133 throughstray capacitance 135 and primary winding 131 to the patient. Therefore,although capacitance 134 between primary and secondary can be madesmall, the leakage current in transformer 130 having earth ground shield133 remains a function of the capacitance between each winding and theshield, i.e., capacitances 135 and 136.

FIG. 4 shows an isolation transformer according to the present inventionwhich has an extremely low capacitance between primary and secondarywindings and noearth ground shield. For convenience in illustration,transformer 51 is shown although transformer 31 has the samecharacteristics as described. The stray capacitance between primarywinding 53 and secondary winding 52 is designated 140 in FIG. 4.According to the present invention, this capacitance is made extremelylow to present an extremely high reactance to low frequency and linefrequency alternating currents. Transformer 51, and similarlytransformer 31, has no earth ground shield and, consequently, there areno stray capacitances between windings 52 and 53 and earth ground.Referring now to FIG. 4, one terminal of primary winding 53 is connectedthrough line 98 to the patients electrical reference point, and theother terminal of primary winding 53 is connected through resistor 109to the circuitry wherein the physiological signal is present. Oneterminal of secondary winding 52 is connected to earth ground, and theother terminal of winding 52 is connected through resistor 111 and othercircuitry to the monitoring equipment.

The absence of an earth ground shield in the isolation transformers ofthe present invention results in the absence of any leakage currentpaths from the primary and secondary windings through stray capacitancesto earth ground. The only stray capacitance providing a path for leakagecurrent is the capacitance between primary and secondary windings, suchas capacitance 140 shown in Fig. 4. According to the present invention,this capacitance is made extremely low to present an extremely highreactance to low frequency and line frequency alternating currents.

Referring now to FIG. 1, circuit 10 operates in the following manner.Physiological signals from a patient undergoing an examination areapplied through leads 15, 16 and 17 to amplifier 11. Amplifier 11, inturn, is energized from d.c. source 20, which delivers an output ofabout 12 volts, and through the electrical path or circuit branchincluding modulator 25, isolation transformer 32, and demodulator 28.The d.c. voltage from source 20 is chopped or modulated in circuit 25,the frequency in this illustration being about 3,000 cycles but in anyevent significantly greater than 60 cycles or line frequency.Transformer 32 includes an extremely low capacitance between primary andsecondary without earth ground shielding to present an extremely highreactance to any low frequency, in particular line frequency,alternating currents which might inadvertently be applied to the path.In addition, the capability of transformer 32 to withstand relativelyhigh voltages applied across the primary and secondary windings withoutbreakdown further precludes the possibility of any electrical shockhazard to the patient through this path.

The physiological signals from the patient then are amplified byamplifier 11 and applied to modulator 41 wherein the amplified signalsare chopped or modulated at the frequency established by circuit 25. Theamplitude modulated signals are transmitted through isolationtransformer 51 to demodulator circuit 44, the output of which isamplified and available at output 50 for utilization by thephysiological monitoring apparatus. Isolation transformer 51 includes anextremely low capacitance between primary and secondary without earthground shielding to present an extremely high reactance to lowfrequency, in particular line frequency, alternating current which mightinadvertently be applied to terminal 50. In addition, the capability oftransformer 51 to withstand relatively high voltages applied across theprimary and secondary windings without breakdown further precludes thepossibility of any electrical shock hazard to the patient from thispath.

Circuit 10 of the present invention thus functions to protect a patientfrom the hazard of leakage currents, which might otherwise be applied tothe patient by the testing equipment or an electrical fault relatedthereto.

The most probable hazard is from standard 60 cycle or line frequencyalternating current which by accident might be present in either path.The relatively low, i.e., less than about 12 picofarad, capacitancebetween the primary and secondary windings of each of the isolationtransformers 32, 51 presents an extremely high reactance to alternatingcurrent at this frequency. This protection afiorded to the patient isenhanced by the fact that each transformer 32, 51 has a relatively highdielectric withstanding voltage.

A circuit 10 constructed according to this illustration of the presentinvention can withstand about 10 kilovolts applied to the input thereofwithout breakdown. In addition, when a source delivering 220 volts rmsat 60 cycles is connected between amplifier input terminals 12, 13 and17 and earth ground, the current flowing through the paths includingisolation transformers 32 and 51 is less than about 5.0 rnicroarnperes.Furthermore, it has been determined that the relationship of leakagecurrent to applied voltage appears to be linear, to values approachingthe dielectric withstanding voltage of transformers 32 and 51.

Circuit 10 thus functions to isolate leakage currents from a patientwhich otherwise would be passed to him if the isolation transformers 32,51 were not present. The operation of circuit 10 utilizes the isolationefi'ect of transformers 32 and 51 along with their minimum capacitancespecifications from primary to secondary without earth ground shielding.A transformer providing leakage protection must of necessity withstandhigh voltage differentials between primary and secondary which in thepresent instance are about 10,000 volts. Another advantageouscharacteristic of circuit 10 is its capability of operating from a dc.voltage source of relatively low magnitude, i.e., source which delivers12 volts. This approach does not necessitate the use of 60 cycle linevoltage for operation. This of course further insures that the patientwill be protected from the possibility of electrical shock. Anotheradvantageous characteristic of circuit 10 is that the electricalreference point of the patient is isolated or floated relative to theequipment electrical ground such as that of the source 20 or of thephysiological monitor ing apparatus connected at terminal 50. Inparticular and referring to FIG. 2, it will be noted that lead 17connected to the patients reference electrode is connected through lead98 to the reference terminals of winding 34 in isolation transformer 32and of winding 53 in isolation transformer 51. In other words,transformers 32 and 51 isolate or float the patients electricalreference point from the earth ground of source 20 and of the apparatusconnected to terminal 50. This same floating or isolation of thepatients reference point is provided by isolation transformers 32 and 51in a multilead'or differential system as previously described inconnection with FIG. 2.

The modulation frequency'utilized in circuit 10 according to thisillustration is about 3,000 cycles per second but in any event issignificantly greater than 60 cycles per second. Experimental evidencehas indicated that the effect of current on the body (except forheating) decreases with increasing frequency. The selection of anoperating frequency for circuit 10 is made from an inspection of therelationship between frequency and let-go current, which is defined asthe highest value of current which still permits the subject to releasea wire. In the frequency range around 60 cycles, this current has thelowest value, beginning to increase relatively slowly in the rangebetween 500 and 1,000 cycles whereupon the rate of increase than becomesmuch steeper.

Circuit 10 of the present invention has been described with particularreference to heartbeat monitoring wherein the physiological signals areelectrocardio signals. It is to be understood, however, that circuit 10can operate effectively with other physiological signals, for exampleblood pressure, electroencephalo, or fetal signals.

It is therefore apparent that the present invention accomplishes itsintended object. While a single embodiment of the present invention hasbeen described in detail, this has been done by way of illustrationwithout thought of limitation.

We claim:

1. A circuit for providing electrical isolation between a patient andphysiological monitoring apparatus comprising:

a. an amplifier; I

b. means for applying a physiological signal from a patient to the inputof said amplifier;

. means defining an electrical path for connecting said amplifier to adc. source of relatively low magnitude; k

. first isolation means in said electrical path;

. means defining a signal transmission path for connecting the output ofsaid amplifier to the input of physiological monitoring apparatus;

f. second isolation means in-said signal transmission path; and saidfirst and second isolation means each including an extremely lowcapacitance and no other leakage current path between the patient andearth ground to present a relatively high reactance to line frequencyalternating current and each being capable of withstanding relativelyhigh voltages applied thereacross without breakdown for isolating thepatient from leakage currents.

2. A circuit according to claim 1 wherein each of said first and secondisolation means comprises a transformer having primary and secondarywindings, the capacitance between said primary and secondary windings ineach transformer being the only stray capacitance providing a path forleakage currents and being sufiiciently low without earth groundshielding to present an extremely high reactance to low frequency andline frequency alternating current so as not to permit a patient hazardfrom leakage currents at low and line frequency.

3. A circuit according to claim 1 wherein said means defining anelectrical path comprises:

a. modulator means for converting a dc. voltage into a periodic signal,the output of which is connected to said first isolation means;

b. means for connecting the input of said modulator to said do. source;and

c. demodulator means having an input connected to said isolation meansand an output connected to said amplifier.

A circuit according to claim 3 wherein the frequency of said periodicsignal is greater than about 1,000 c.p.s.

5. A circuit according to claim 3 wherein said first isolation meanscomprises a transformer having a primary winding connected to the outputof said modulator means and a secondary winding connected to the inputof said demodulator means, the capacitance between said primary andsecondary windings being the only stray capacitance providing a path forleakage currents and being sufficiently low without earth groundshielding to present an extremely high reactance to low frequency andline frequency altemating current so as not to permit a patient hazardfrom leakage currents at low and line frequency.

6. A circuit according to claim 3 further including means connecting theoutput of said modulator means to said signal transmission path forsynchronizing the operation of said signal transmission path with theoperation of said electrical path.

7. A circuit according to claim 1 wherein said means defining a signaltransmission path comprises:

a. modulator means for producing a periodic signal, the output of whichis connected to said second isolation means;

b. means for connecting the input of said modulator to the output ofsaid amplifier;

c. demodulator means having an input connected to said second isolationmeans and an output; and

d. means for coupling the output of said demodulator means to saidphysiological monitoring apparatus.

8. A circuit according to claim 7 wherein said second isolation meanscomprises a transformer having a primary winding connected to the inputof said demodulator means and a secondary winding connected to theoutput of said modulator means, the maximum capacitance between saidprimary and secondary windings being the only stray capacitanceproviding a path for leakage currents and being sufficiently low withoutearth ground shielding to present an extremely high reactance to lowfrequency and line frequency alternating current so as not to permit apatient hazard from leakage currents at low and line frequency.

9. A circuit according to claim 7 wherein the frequency of said periodicsignal is greater than about 1,000 c.p.s.

10. A circuit according to claim 7 further including means connectingsaid modulator means and said demodulator means to said electrical pathfor synchronizing the operation of said modulator means and demodulatormeans with that of said electrical path.

11. A circuit for providing electrical isolation between a patient andphysiological monitoring apparatus comprising:

a. an amplifier having an input and an output;

b. means for applying a physiological signal from a patient to the inputof said amplifier;

c. a first circuit branch comprising a modulator, an isolationtransformer connected to said modulator, and a demodulator connected tosaid transformer, the output of said demodulator being connected to saidamplifier;

d. means for connecting the input of said modulator to a dc. voltagesource of relatively low magnitude;

e. a second circuit branch comprising a modulator,

an isolation transformer connected to said modulator, and a demodulatorconnected to said transformer, the input of said modulator beingconnected to the output of said amplifier;

f. means for coupling the output of said demodulator in said secondbranch to the input of physiological monitoring apparatus; and

g. each of said isolation transformers including an extremely lowcapacitance between the primary and secondary windings thereof withoutearth ground shielding to present a relatively high reactance to lowfrequency, including line frequency, altemating current and each beingcapable of withstanding relatively high voltages applied across thewindings thereof without breakdown.

12. A circuit according to claim 11 wherein the capacitance between saidprimary and secondary windings is the only stray capacitance providing apath for leakage currents and is sufficiently low so as not to permit apatient hazard from leakage currents at low and line frequency.

A circuit accordmg to claim 11 wherein said modulators in said first andsecond circuit branches each operate at a frequency substantiallygreater than line frequency.

14. A circuit according to claim 11 further including means connectingthe output of said modulator in said first circuit branch to saidmodulator and demodulator in said second circuit branch whereby theoperation of the latter are synchronized with the frequency ofmodulation in said first branch.

15. A circuit according to claim 11 wherein said modulator in saidsecond branch comprises a switching transistor connected between saidmodulator input and a ground or reference potential and having a controlterminal connected to said first circuit branch whereby amplifiedphysiological signals applied to the input of said modulator are choppedat a rate equal to the frequency of modulation of said modulator in saidfirst branch.

16. A circuit according to claim 11 wherein said coupling meanscomprises an amplifier having an RC network connected across the inputand output thereof and wherein said demodulator in said second branchcomprises a transistor switch connected between said isolationtransformer and said coupling means and having a control terminalconnected to said modulator in said first circuit branch.

1. A circuit for providing electrical isolation between a patient andphysiological monitoring apparatus comprising: a. an amplifier; b. meansfor applying a physiological signal from a patient to the input of saidamplifier; c. means defining an electrical path for connecting saidamplifier to a d.c. source of relatively low magnitude; d. firstisolation means in said electrical path; e. means defining a signaltransmission path for connecting the output of said amplifier to theinput of physiological monitoring apparatus; f. second isolation meansin said signal transmission path; and g. said first and second isolationmeans each including an extremely low capacitance and no other leakagecurrent path between the patient and earth ground to present arelatively high reactance to line frequency alternating current and eachbeing capable of withstanding relatively high voltages appliedthereacross without breakdown for isolating the patient from leakagecurrents.
 2. A circuit according to claim 1 wherein each of said firstand second isolation means comprises a transformer having primary andsecondary windings, the capacitance between said primary and secondarywindings in each transformer being the only stray capacitance providinga path for leakage currents and being sufficiently low without earthground shielding to present an extremely high reactance to low frequencyand line frequency alternating current so as not to permit a patienthazard from leakage currents at low and line frequency.
 3. A circuitaccording to claim 1 wherein said means defining an electrical pathcomprises: a. modulator means for converting a d.c. voltage into aperiodic signal, the output of which is connected to said firstisolation means; b. means for connecting the input of said modulator tosaid d.c. source; and c. demodulator means having an input connected tosaid isolation means and an output connected to said amplifier. ,4 Acircuit according to claim 3 wherein the frequency of said periodicsignal is greater than about 1,000 c.p.s.
 5. A circuit according toclaim 3 wherein said first isolation means comprises a transformerhaving a primary winding connected to the output of said modulator meansand a secondary winding connected to the input of said demodulatormeans, the capacitance between said primary and secondary windings beingthe only stray capacitance providing a path for leakage currents andbeing sufficiently low without earth ground shielding to present anextremely high reactance to low frequency and line frequency alternatingcurrent so as not to permit a patient hazard from leakage currents atlow and line frequency.
 6. A circuit according to claim 3 furtherincluding means connecting the output of said modulator means to saidsignal transmission path for synchronizing the operation of said signaltransmission path with the operation of said electrical path.
 7. Acircuit according to claim 1 wherein said means defining a signaltransmission path comprises: a. modulator means for producing a periodicsignal, the output of which is connected to said second isolation means;b. means for connecting the input of said modulator to the output ofsaid amplifier; c. demodulator means having an input connected to saidsecond isolation means and an output; and d. means for coupling theoutput of said demodulator means to said physiological monitoringapparatus.
 8. A circuit according to claim 7 wherein said secondisolation means comprises a transformer having a primary windingconnected to the input of said demodulator means and a secondary windingconnected to the output of said modulator means, the maximum capacitancebetween said primary and secondary windings being the only straycapacitance providing a path for leakage currents and being sufficientlylow without earth ground shielding to present an extremely highreactance to low frequency and line frequency alternating current so asnot to permit a patient hazard from leakage currents at low and linefrequency.
 9. A circuit according to claim 7 wherein the frequency ofsaid periodic signal is greater than about 1,000 c.p.s.
 10. A circuitaccording to claim 7 further including means connecting said modulatormeans and said demodulator means to said electrical path forsynchronizing the operation of said modulator means and demodulatormeans with that of said electrical path.
 11. A circuit for providingelectrical isolation between a patient and physiological monitoringapparatus comprising: a. an amplifier having an input and an output; b.means for applying a physiological signal from a patient to the input ofsaid amplifier; c. a first circuit branch comprising a modulator, anisolation transformer connected to said modulator, and a demodulatorconnected to said transformer, the output of said demodulator beingconnected to said amplifier; d. means for connecting the input of saidmodulator to a d.c. voltage source of relatively low magnitude; e. asecond circuit branch comprising a modulator, an isolation transformerconnected to said modulator, and a demodulator connected to saidtransformer, the input of said modulator being connected to the outputof said amplifier; f. means for coupling the output of said demodulatorin said second branch to the input of physiological monitoringapparatus; and g. each of said isolation transformers including anextremely low capacitance between the primary and secondary windingsthereof without earth ground shielding to present a relatively highreactance to low frequency, including line frequency, alternatingcurrent and each being capable of withstanding relatively high voltagesapplied across the windings thereof without breakdown.
 12. A circuitaccording to claim 11 wherein the capacitance between said primary andsecondary windings is the only stray capacitance providing a path forleakage currents and is sufficiently low so as not to permit a patienthazard from leakage currents at low and line frequency.
 13. A circuitaccording to claim 11 wherein said modulators in said first and secondcircuit branches each operate at a frequency substantially greater thanline frequency.
 14. A circuit according to claim 11 further includingmeans connecting the output of said modulator in said first circuitbranch to said modulator and demodulator in said second circuit branchwhereby the operation of the latter are synchronized with the frequencyof modulation in said first branch.
 15. A circuit according to claim 11wherein said modulator in said second branch comprises a switchingtransistor connected between said modulator input and a ground orreference potential and having a control terminal connected to saidfirst circuit branch whereby amplified physiological signals applied tothe input of said modulator are chopped at a rate equal to the frequencyof modulation of said modulator in said first branch.
 16. A circuitaccording to claim 11 wherein said coupling means comprises an amplifierhaving an RC network connected across the input and output thereof andwherein said demodulator in said second branch comprises a transistorswitch connected between said isolation transformer and said couplingmeans and having a control terminal connected to said modulator in saidfirst circuit branch.